Infrared spectroscopy of liquids is useful for characterizing liquid components. Different liquid or dissolved chemicals have strong identifying absorption features in the infrared wavelengths.
Traditionally, Fourier transform infrared (FTIR) spectrometers have been used for liquid characterization. However, liquids present unique challenges for FTIR spectroscopy. First, most liquids have strong background absorptions. Because the optical powers per wavelength available for FTIR spectrometers are quite low due to the use of broadband globar incandescent source, the path lengths through liquids that can be probed are quite small before the probe light is attenuated to unacceptably low values. Hence, FTIR is typically used to determine percent level fractions of components in liquids, and not trace additions (less than one part per thousand) to liquids that would require longer liquid path lengths for adequate sensitivity. Also, this has pushed FTIR spectroscopy to use attenuated total reflectance (ATR) interfaces. These interfaces typically result in smaller path lengths, and have the problem that they distort the spectral signatures of the chemicals being probed due to a combined effect of absorption and changing refractive index on the signal. They are therefore not well suited to quantitative liquid spectroscopy, or trace detection.
Alternatively, transmission liquid spectroscopy through a flow cell is another technique for quantitative liquid spectroscopy and trace detection. Unfortunately, existing flow cells and existing transmission liquid spectroscopy systems are not adequate.